We each have those few things that we feel we cannot live without. Well, perhaps we could live without them- but preferably in a cave or behind really BIG sunglasses and a wide brimmed hat.
So what's yours? Why? And what type/brand?

My eyelashes are pretty light blonde. I am naturally a redhead.
Mascara will instantly enhance my eyes and make them stand out so much. I rarely leave the house without wearing mascara.

For a natural look that is just the right amount of darkness and color, I usually use Bare Esscentuals Big Tease in black/brown. It has a neat brush- one side is long for great coverage and definition on your upper lashes; the other side is short and perfect for your lower lashes. Very natural looking, and slightly lengthening.
For more dramatic, I use Dior's Diorshow in Black. That stuff is intense, and allows me to do my 1960's doe eye look very well (coupled with liquid eyeliner, of course.)